According to a known technique the movement of a stream of material can be accelerated with the aid of centrifugal force. With this technique the material is fed onto the central surface of a rotor and is then picked up by guide members which are arranged around said central surface and are carried by said rotor. The material is accelerated along the guide members, under the influence of centrifugal forces, and propelled outwards at high velocity and at a certain take-off angle. The velocity that the material acquires during this operation is made up of a radial velocity component and a velocity component oriented perpendicularly to the radial, or transverse velocity component. Viewed from the stationary position, the material moves at virtually constant velocity along a virtually straight as after it has left the guide member. This straight stream is directed forwards, viewed in the direction of rotation, and the magnitude of the take-off angle is in this case determined by the magnitudes of radial and transverse velocity components. If these components are identical the take-off angle is 45°. Viewed from a standpoint moving with the guide member the material moves in a spiral stream after it leaves the guide member, which spiral stream is directed backwards, viewed in the direction of rotation, and is in the extension of the release end of the guide member. In this case the relative velocity increases along said spiral path.
Guiding can take place along a metal guide surface that is oriented radially outwards. Such a guide surface is disclosed in U.S. Pat. No. 5,184,784. Autogenous guiding is also possible, along a so-called dead or autogenous bed of own material that under the influence of centrifugal force settles as a continuous layer in a chamber member that is arranged along the edge of the rotor. An autogenous rotor of this type is disclosed in U.S. Pat. No. 4,940,188 and is of particular importance with regard to the autogenous rotor according to the invention. In the known autogenous rotor the chamber member is provided with a chamber wall that is at least partially arranged tangentially and in any event does not extend in the radial direction. As a result of this tangential arrangement no, or only limited, movement forces are able to develop along the chamber wall, with the consequence that the material settles on the chamber wall. However, the chamber wall extends—increasingly radially oriented—towards the outer edge of the rotor, with the consequence that (radial) acceleration forces gradually build up towards the outside, which cause the material to move along the autogenous granular bed towards the outside. At the end of the chamber wall there is a tip over which the material is propelled outwards from the rotor, the take-off velocity being essentially determined by the transverse velocity component.
Many shapes of chamber members are conceivable and known. For instance, instead of a tangential wall, the autogenous bed can also be built up in contact with a circular chamber wall, in which case the material settles as it were in a bowl. A rotor of this type is disclosed in U.S. Pat. No. 4,575,014 and U.S. Pat. No. 1,405,151.
It is also possible to construct the rotor with symmetrical chamber members. Such a rotor is disclosed in JP 08266920. This solution has the advantage that the rotor can be rotated in both directions, as a result of which the life time of the rotor, that essentially is determined by the number of tips, is doubled.
The material propelled outwards can now be collected by a stationary impact member that is arranged in the straight sin that the material describes, with the aim of causing the material to break during the impact. The comminution process takes place during this single impact, the equipment being referred to as a single impact crusher. The stationary impact member can, for example, be formed by an armoured ring, which is arranged around the rotor. Such a device is disclosed in U.S. Pat. No. 4,690,341. It is also possible to allow material impinge autogenously on a bed of own material. Such a device is disclosed in U.S. Pat. No. 4,662,571.
Instead of allowing the material to impinge directly on a stationary impact member, it is also possible first to allow the material to impinge on an impact member that is co-rotating with the guide member and that is rotating at the same velocity, in the same direction and about the same axis of rotation, but a greater radial distance away from said axis of rotation than is said guide member, and is arranged transversely in the spiral stream which the material describes. Such equipment is referred to as a direct multiple impact crusher. Because the impact with the co-rotating impact member takes place essentially deterministically, the impact surface can be arranged at an angle such that the impact takes place at an optimum angle. Such a method and device are disclosed in PCT/NL 97/00565, which was drawn up in the name of the Applicant.
EP 1 084 751 A1, which was drawn up in the name of the Applicant, discloses a symmetrical rotor that is provided with guide members and associated impact members, a facility being provided for making the impact members partially autogenous.
The known autogenous rotor by means of which the material moves over an autogenous bed of material in the direction of the tip and from there is propelled outwards from the rotor has the advantage that wear is limited, compared with a rotor where the material is accelerated along a (more radially oriented) steel guide surface. However, the known autogenous rotors also have disadvantages. For instance, fairly substantial wear still occurs along the tip, certainly in the case of more abrasive material. Another (major) disadvantage is that the material, when it is metered onto the central surface of the rotor and moves (abrasively) outwards over the rotor blade, moves, relative to the autogenous bed, in a (spiral) direction that is opposed to the direction of rotation of said autogenous bed. In order to be picked up by the autogenous bed and then to be guided along said autogenous bed towards the edge of the rotor (tip), the direction of movement of the material must therefore be reversed through approximately 180°. This costs a great deal of energy, results in substantial wear on the rotor blades and is the reason why the flow of the material is hindered, as a result of which the capacity is substantially restricted. As a result of the reversal of the stream of material a certain degree of comminution (grinding) of the material takes place as a result of mutual friction (attrition) of the grains. This can give rise to an excess of fine particles. Furthermore, the chamber members take up a fairly large amount of space, as a result of which the space in which the material can flow through is restricted. The rotor can therefore usually be constructed with a maximum of three chamber members, which are of symmetrical or non-symmetrical construction. This limits the life time, which, incidentally, is mainly determined by the tips. Another disadvantage is that the material must not be too wet or sticky because the rotor can then clog; in any event the throughput is substantially impeded. Furthermore, the maximum grain diameter that can be processed is usually restricted to 40–50 mm.